We have focused in three related areas: 1. the study of vFLIP, a KSHV latent gene product expressed in KSHV-infected cell targets and in Kaposi's sarcoma (KS), Primary Effusion Lymphoma (PEL) and Multicentric Castleman's disease;2. the study of CXCR7, a G protein-coupled receptor induced by KSHV in the host cells, and its related receptor CXCR4, which is not induced by KSHV;and 3. the development of new therapies for KSHV-induced malignancies occurring in AIDS patients. One of the characteristic features of KSHV is its ability to infect endothelial cells,and to indirectly promote angiogenesis and lymphangiogenesis predominantly by promoting the recruitment of cells that produce pro-angiogenic factors and promoting the expression of pro-angiogenic genes by the cells it infects. ORFK13/vFLIP encodes a 188-amino acid protein, which binds to the Ikb kinase (IKK) complex to activate NFkB. We examined ORFK13/vFLIP contribution to KS phenotype and potential for therapeutic targeting. To this end, we have retrovirally transduced ORFK13/vFLIP into primary human endothelial cells and examined the contribution of this gene to KS phenotype. We found that ORFK13/vFLIP induces the spindle morphology distinctive of KS cells and promotes formation of abnormal vascular networks typical of the disorderly KS vasculature. Microarray analysis of gene expression in endothelial cells transduced with ORFK13/vFLIP detected increased expression of pro-inflammatory cytokines, chemokines, and interferon-responsive genes. This study represents the first comprehensive analysis of gene regulation by KSHV-vFLIP. As one might expect from stimulation of pro-inflammatory cytokines and chemokines, we found that ORFK13/vFLIP stimulates adhesion of inflammatory cells characteristic of KS lesions. In additional experiments, we found that that KSHV K13 induces the expression of the NF-kB regulatory proteins A20, ABIN-1 and ABIN-3 in primary human microvascular endothelial cells, and that KS spindle cells express A20 in KS tissue. In reporter assays, A20 strongly impaired K13-induced NF-kB activation in 293T cells, but ABIN-1 and ABIN-3 did not. Mutational analysis established that the C-terminal domain (residues 427-790) is critical for A20 modulation of NF-kB. In functional assays, A20 inhibited K13-induced secretion of IP-10, and reduced K13-induced cell proliferation. Thus, we demonstrate that A20 negatively regulates NF-kB activation directly induced by KSHV K13. By attenuating excessive and prolonged NF-kB activation induced by K13 that could be harmful to KSHV-infected cells, A20 likely plays an important role in the pathogenesis of KSHV-associated diseases, in which K13 is expressed. One of the cellular genes that are highly induced by KSHV is the chemokine receptor RDC1/CXCR7. Recent studies have shown that CXCR7 binds the chemokines SDF1 and I-TAC but it is still unclear whether CXCR7 can signal in response to these ligands or other signals, or whether its function is to serve to sequester ligands away from their receptors. Recently, CXCR7 was shown to oligomerize with CXCR4, a receptor that can signal in response to SDF1. We are interested in the function of CXCR7 in the context of KSHV infection. We have overexpressed or silenced CXCR7 in PEL (Primary Effusion Lymphoma) cell lines and tested their tumorigenicity in mice. Initial observations have shown that CXCR7 promotes PEL-induced tumor progression. We are currently exploring the mechanisms underlying this pro-tumorigenic effect of CXCR7 in the context of PEL malignancy. PEL is a fatal viral malignancy in humans, which typically presents as a malignant effusion that later disseminates. In spite of therapy with high-dose chemotherapy or other therapies, PEL is a rapidly fatal malignancy. Rapamycin, which targets mTOR (mammalian target of rapamycin), an effector of cell signaling pathways often deregulated in cancer, showed efficacy against a variety of tumors, particularly those of lymphoid origin. We have investigated the potential utility of Rapamycin for the treatment of experimental PEL. Previous studies have suggested that rapamycin could be effective against subcutaneous PEL in mice. However, this pre-clinical model is far removed from the disease in patients in its location and progression. Recently, PEL development in rapamycin-treated post-transplant recipients raised questions about the drug's anti-PEL activity. We have developed and used a murine model of effusion PEL progressing to peritoneal tumors to investigate the anti-PEL activity of rapamycin. We found that rapamycin significantly reduces ascites accumulation and extends mouse survival. Initially, rapamycin reduced PEL load compared to control mice, but most mice rapidly showed PEL progression. Levels of VEGF, which promotes vascular permeability contributing to effusion formation, were significantly reduced in ascites of rapamycin-treated mice compared to controls. Expression of IL-10, the principal autocrine growth factor for PEL, was initially reduced in PEL from rapamycin-treated mice but rapidly increased despite treatment. We found that the hypoxic environment of ascites and rapamycin cooperate in stimulating IL-10 expression in PEL. These results do not support the use of rapamycin as a curative treatment for PEL, but identify rapamycin an effective drug to reduce accumulation of malignant effusions. Current efforts in the laboratory are intended to further characterize development of PEL resistance to rapamycin and how to prevent it. In particular, we are testing the potential efficacy of combining rapamycin to neutralization of IL-10. In other experiments we have examined the biochemical basis for diversity of phenotype within KS cells that are KSHV-infected. Such diversity has created unresolved uncertainties as to the origin of KS tumor cells. We have examined the possibility that KSHV infects endothelial cells and turns them into mesenchymal cells. Endothelial to mesenchymal transition (EndMT), the process by which endothelial cells convert into mesenchymal cells, plays critical roles during development of the heart, and underlies certain forms of pathological organ fibrosis and tissue ossification. We found that Kaposis sarcoma-associated herpesvirus (KSHV) is an inducer of EndMT. Upon KSHV infection, primary dermal microvascular endothelial cells lose expression of endothelial markers, acquire expression of mesenchymal markers, display new invasive and migratory properties, and exhibit increased survival. We discovered that the canonical Notch signaling pathway and the Notch-induced transcription factors Slug and ZEB1 are deployed by KSHV to induce activation of EndMT, whereas the TGFb signaling pathway previously linked to EndMT, is not utilized. The KSHV-infected spindle cells within KS lesions display a complex phenotype with features of endothelial and mesenchymal cells, display evidence of Notch activity and express nuclear ZEB1, features compatible with KSHV-induced EndMT in vivo. These results show that KSHV utilizes the EndMT program to endow endothelial cells with invasiveness and resistance to death, traits that likely contribute to KS progression and KSHV persistence. Targeting Notch signaling emerges as a novel experimental approach to the treatment of KS.